Based on work by Jon Challinger (see earlier commit)

This allows the GCS to display to the user where the vehicle is flying
to in RTL and Guided flight modes

This reduces the chance that a forgotten rally point will cause the
vehicle to RTL to a distant location because instead it will RTL to
home.

RGB LED will remain flashing blue when vehicle is disarmed and this
check has failed (i.e. false).

Also changed mode_allows_arming function to accept arming_from_gcs param
Also remove AUTOTUNE from arming list

Also initialise throttle_zero  to true on startup
Treat throttle less than zero as zero

this prevents a corrupted microSD card from causing a continuous
attempt to open a log file while in flight, which can cause large
scheduler delays

Pair-Programmed-With: Grant Morphett <grant@gmorph.com>

the fsync just wastes time reopening /fs/mtd

This enables user to set the external parameter to true even if only one compass is connected